Short Notes - Force & Laws of Motion - Class 9 PDF Download

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Class IX: Science 
Chapter 2: Force and Laws of Motion 
Chapter Notes 
 
Key Learning: 
1. Force is a push or pull acting upon an object. 
 
2. Balanced forces: The resultant of all the forces acting on a body is zero. 
 
3. Unbalanced forces: The resultant of all the forces acting on a body is not 
zero. 
 
4. Newton’s first law of motion states that A body at rest will remain at rest 
and a body in motion will remain in uniform motion unless acted upon by an 
unbalanced force. 
 
5. The property by the virtue of which an object tends to remain in the state 
of rest or of uniform motion unless acted upon by some force is called inertia. 
 
6. The mass of a body is a measure of inertia. 
 
7. The momentum of an object is the product of its mass and velocity and 
has the same direction as that of the velocity. Its SI unit is kg m s
-
¹. 
 
8. Newton’s second law of motion states that the rate of change of 
momentum of a body is directly proportional to the force and takes place in 
the same direction as the force. 
 
9. Force is also defined as the product of mass and acceleration. 
  
10. The SI unit of force is kg m s
-
². This is also known as newton and 
represented by the symbol N. A force of one newton produced an 
acceleration of 1 m s
-
² on an object of mass 1 kg. 
 
11. Force of friction always opposes motion of objects. 
 
12. Two forces resulting from the interaction between two objects are called 
action and reaction forces respectively. 
 
13. Action and reaction forces act on two different bodies but they are equal 
in magnitude. 
Page 2


 1 
Class IX: Science 
Chapter 2: Force and Laws of Motion 
Chapter Notes 
 
Key Learning: 
1. Force is a push or pull acting upon an object. 
 
2. Balanced forces: The resultant of all the forces acting on a body is zero. 
 
3. Unbalanced forces: The resultant of all the forces acting on a body is not 
zero. 
 
4. Newton’s first law of motion states that A body at rest will remain at rest 
and a body in motion will remain in uniform motion unless acted upon by an 
unbalanced force. 
 
5. The property by the virtue of which an object tends to remain in the state 
of rest or of uniform motion unless acted upon by some force is called inertia. 
 
6. The mass of a body is a measure of inertia. 
 
7. The momentum of an object is the product of its mass and velocity and 
has the same direction as that of the velocity. Its SI unit is kg m s
-
¹. 
 
8. Newton’s second law of motion states that the rate of change of 
momentum of a body is directly proportional to the force and takes place in 
the same direction as the force. 
 
9. Force is also defined as the product of mass and acceleration. 
  
10. The SI unit of force is kg m s
-
². This is also known as newton and 
represented by the symbol N. A force of one newton produced an 
acceleration of 1 m s
-
² on an object of mass 1 kg. 
 
11. Force of friction always opposes motion of objects. 
 
12. Two forces resulting from the interaction between two objects are called 
action and reaction forces respectively. 
 
13. Action and reaction forces act on two different bodies but they are equal 
in magnitude. 
 2 
 
14. Newton’s third law: For every action there is an equal and opposite 
reaction; but action and reaction acts on different bodies. 
 
15. Law of conservation of momentum: The sum of momenta of the two 
objects before collision is equal to the sum of momenta after the collision 
provided there is no external unbalanced force acting on them. 
 
16. The velocity with which gun moves in the backward direction is known as 
the recoil velocity. 
 
Top Formulae: 
1. Momentum p = m v;  
m = mass of the body; v = velocity of the body 
2. Force 
dp
F
dt
? 
3. Force F = m a; where a = acceleration 
4. Law of conservation of momentum 
 Total momentum before collision = Total momentum after collision  
1 1 2 2 1 1 2 2
m u m u m v m v ? ? ? 
 Where m
1
, m
2
: masses of the bodies 
  u
1
, u
2
: initial velocities of bodies 
  v
1
, v
2
: final velocities of bodies